SEPolicy for boringssl_self_test.

This CL adds hand-written SELinux rules to:
 - define the boringssl_self_test security domain
 - label the corresponding files at type boringssl_self_test_marker
   and boringssl_self_test_exec.
 - define an automatic transition from init to boringssl_self_test
   domains, plus appropriate access permissions.

Bug: 137267623
Test: When run together with the other changes from draft CL topic
      http://aosp/q/topic:bug137267623_bsslselftest, check that:
      - both /dev/boringssl/selftest/* marker files are
        present after the device boots.
      - Test: after the boringssl_self_test{32,64} binaries have
        run, no further SELinux denials occur for processes
        trying to write the marker file.

Change-Id: I77de0bccdd8c1e22c354d8ea146e363f4af7e36f
diff --git a/apex/com.android.conscrypt-file_contexts b/apex/com.android.conscrypt-file_contexts
index ffc3109..abf0085 100644
--- a/apex/com.android.conscrypt-file_contexts
+++ b/apex/com.android.conscrypt-file_contexts
@@ -1,5 +1,6 @@
 #############################
 # System files
 #
-(/.*)?                   u:object_r:system_file:s0
-/lib(64)?(/.*)?          u:object_r:system_lib_file:s0
+(/.*)?                          u:object_r:system_file:s0
+/lib(64)?(/.*)?                 u:object_r:system_lib_file:s0
+/bin/boringssl_self_test(32|64) u:object_r:boringssl_self_test_exec:s0